Lithographic methods are at the heart of modern day microfabrication, nanotechnology and molecular electronics. Microfabrication techniques such as photolithography, microcontact printing, micromachining, and microwriting can produce patterns as small as 100 nm, but the production of sub-100 nm structures still poses a challenge. Amro et al., Langmuir, 16, 3006-3009 (2000). At present, such high-resolution fabrication can be achieved using lithography techniques and a variety of scanning probe lithography (SPL) techniques have been developed for this purpose.
One such technique is dip pen nanolithograpy (DPN). See, e.g., Piner et al., Science, 283, 661-663 (1999); Hong et al., Science, 286, 523-528 (1999); Weinberger et al. Advanced Materials, 12, 1600-1603 (2000); and PCT application WO 00/41213. DPN is a nanolithography technique by which molecules are directly transported to a substrate of interest in a positive printing mode. DPN utilizes a solid substrate as the “paper” and a scanning probe microscope (SPM) tip (e.g., an atomic force microscope (AFM) tip or a near field scanning optical microscope (NSOM) tip) as the “pen.” The tip is coated with a patterning compound (the “ink”), and the coated tip is used to apply the patterning compound to the substrate to produce a desired pattern. DPN enjoys numerous advantages for depositing “nanoscale” wide mono- or multilayer molecules. The DPN delivery mechanism may involve the formation of a meniscus around the SPM tip and the control of the movement of the patterning molecules to the surfaces on which they are deposited by a driving force. Considerable recent work done in this area demonstrates the efficacy of the DPN approach to pattern monolayer molecules at the lateral width which is limited only by the liquid meniscus. DPN is described in pending application Ser. No. 09/477,997, filed Jan. 5, 2000, the complete disclosure of which is incorporated herein by reference.
Problems that arise with DPN technology stem from the dependence of the technique on the liquid meniscus. For example, the lateral width of the line written by the “pen” using DPN technology is limited by the width of the meniscus formed. The meniscus is subject to variations in the relative humidity as well as chemical interactions between the solvent and the substrate. The size of the meniscus also affects the rate of the transport of the patterning compound to the substrate. This may require coating of the microscope tip with hydrophobic compounds if the nanolithography is to be performed in air. Solubility characteristics of the “ink” molecules in a given solvent can create difficulty in establishing a desired line width and a suitable loading concentration of the ink in the solvent. Furthermore, surface tension characteristics of different solvents can lead to drip or rapid flow from the pen leading to problems with precise control of the ink application under some circumstances. Accordingly, additional and improved lithography techniques that could overcome these problems and extend the application to pattern with whiskers, clusters and nanocrystals as well as increased rates of deposition would be highly desirable for use in a variety of fields.